A screw is a bolt (or pin), which may have a drive, wherein the bolt is provided with a screw thread at the outer side. In screws having a head, the annular area under the head serves as an abutment against the part, which is to be fixed to it.
From EP 0 589 398 B1 and/or DE 10 2010 028 344 A1 it is known for an underground of plastics or of a light metal to screw a screw having a screw thread into the underground (or subfont), wherein non-cylindrical core sections, which narrow (or taper) in the direction towards the screw head, are arranged between the thread turns. Thereby, the material of the underground of plastics or light metal, which material has been pushed aside (or displaced) during the screwing-in, is to flow only in a rear side direction along each core section up to the region of the core section having the smallest diameter, in order to enable a reduced screwing torque of the screw during the screwing-in after having a drilled a pilot hole which has been performed previously as a general rule.
For the introduction of a screw into an underground made of plastics it is stipulated to drill a pilot hole into the plastics underground and/or to provide this during the injection-moulding (primary shaping) and to subsequently screw-in a screw into the hole. The hole is pilot-drilled with a diameter which is larger than the shaft diameter of the screw. Thereby, sections of a screw for a plastics underground may have, between neighbouring thread turns, a shape which deviates from a cylinder geometry. In a screw from the company Reyher, which is referred to as RST, and which is for screwing-in into a pilot-drilled plastics underground, sections of the screw, between neighbouring thread turns, may be formed narrowing (or tapering) away from the screw head towards the opposite screw end. Demonstratively, the high thread turns of such screws for plastic undergrounds cut into the plastic material, which adjoins the pilot-drilled drill hole. Stated differently, the anchoring effect of such screws for plastic undergrounds is based solely on the anchorage of the thread turns in the pilot-drilled underground, whereas the screw shaft is standing substantially free in the pilot-drilled drill hole.
Totally different technical principles and physical framework conditions as compared to undergrounds of plastics or light metal apply to the introduction of a screw into wood. When introducing a screw into wood, the formation of a pilot drill hole is relinquished as a general rule; rather, a wood screw is typically screwed-in into a massive wood underground without drilling a pilot hole. Thereby, the screw thread of a wood screw cuts itself its counter-thread in the wood underground. An example of a known wood screw is the screw Assy Plus or Assy 3.0, which are marketed by the company Würth. The extrication value is a measure for the quality of the anchorage of a wood screw in a wood underground. Hereby, the drag force is referred to, which is necessary for pulling the wood screw out of the wood underground, thus for surmounting the holding force of the wood screw in the wood underground.
Even if the known wood screw of the type Assy Plus and/or Assy 3.0 exhibits already very good extrication values, a further improved extrication resistance of a wood screw is nevertheless desirable. Of course, such a screw shall have favourable properties in respect of the manufacturability.
DE 195 25 732 discloses a screw having a screw head, a screw shaft which has a screw thread as well as a screw tip section which has at least one cutting (or milling) rib, which forms a cutting edge that projects over the surface of the screw tip section.
EP 0 939 235 discloses a screw having a shaft part and a tip as well as at least one blade edge in the tip section, wherein a screw thread extends at least over partial lengths of the shaft part and the tip, wherein the blade edge is formed on at least one rib, which projects beyond the core diameter of the shaft part and has a smaller inclination angle with respect to the longitudinal axis of the screw than the screw thread, and wherein the screw thread extends over the total length of the tip up to the pointed end of the screw, wherein the tip has a cone-shaped end section, which is free of ribs, and over which the screw thread continues (or proceeds) continuously.